if in /path/first/ exists folder N
then seriesFormat should be "path/first/N/F"
else if in /path/second/ exists folder N
then seriesFormat should be "path/second/N/F"
else
seriesFormat will be as usual "path/third/N"

Sorry, I need only check existence of folders "path/first/N" and "path/second/N".
How to read this: a.diskSpace <=> b.diskSpace?
There is no need in checking disk space. We should select path ONLY on the fact of existence of the folders (in order):
if (N exists in first)
we select path/first/N
else if (N exists in second)
we select path/second/N
else
we select path/third/N
I need a string for filebot-node seriesFormat in Synology DSM WITHOUT plex server. Can I use plex in expressions? Are the def's you specified do the same as my long expressions?

Is it possible to refer to "Output Folder" specified in FileBot-Node? Or I must use absolute path inside [.., .., ..].?
def N gives not the same I wrote initially. But this not a problem.
The expression don't works. It always give me first part (/path/1st/N above) but this folder not exists. In this case it should check second part (existence /path/2nd/N) and return it (if exists /path/2nd/N) or /path/3rd/N (if /path/2nd/N not exists).